Electrography refers to the processes of electrophotography, electroradiography, and magnetography. The process of electrography has been described in numerous patents, such as those issued to Chester F. Carlson, including U.S. Pat. Nos. 2,221,776, 2,297,691, and 2,357,809. The process, as taught in these and other patents, essentially comprises production of a latent electrostatic image using photoconductive media and the subsequent development and transfer of a visible image therefrom. A latent electrostatic image may also be formed by spraying the charge onto a suitable charge-retaining surface as taught, for example, in U.S. Pat. Nos. 2,143,214, 3,773,417, and 3,017,560. In magnetography, the latent image is magnetic and may be developed with appropriately magnetized or magnetizable developer particles, as described in U.S. Pat. No. 3,520,811.
Development of the latent image can be accomplished by deposition of developer particles on the electrostatic or magnetic latent image, the most common technique using powder or cascade development, but liquid developers are also utilized in the prior art. A liquid developer comprises a dispersion of the developer particles in a suitable liquid dispersant.
Transfer of the developed image to another surface is often accomplished by means of externally applied electrostatic forces, by adhesion of the image particles to a "tacky" receptor sheet using contact and pressure, or by utilization of a resin-coated receptor sheet having a desirable transfer surface. Fixing of the transferred image is frequently accomplished by pressure, heating, and subsequently cooling to room temperature.
Starting with an image which has been freshly developed with liquid toner (dispersant is still present), transfer and fixation may be accomplished by absorption and/or electrostatic transfer, as disclosed in U.S. Pat. Nos. 3,419,411, 3,247,007, and 2,899,335. Where dispersant is still present upon transfer, the images may suffer from the problem of lateral displacement. Such a problem prevents good resolution of the image. If it is desirable to remove the dispersant, the additional problems of evaporation, heat, and safe removal of vapors are present. U.S. Patent Office Defensive Publication No. T879,009 discloses a receiving sheet with a softened surface which is pressed against an organic photoconductor bearing a liquid developed xerographic image which image retains a portion of the liquid developing solvent therein. The image transfers to the receiver during the application of heat and pressure. About 10 to 95% of the liquid developer solvent is removed, with at least 5% residual solvent required for transfer. The receiver sheets are coated with solvent-susceptible resins, which apparently "swell" in the presence of the liquid dispersant and allow the toner particles to become imbedded in the resin coating. The resin coating weight is 0.2 g/ft.sup.2 (about 1.5 microns thick) compared to the preferred thicker coatings (about 3 to 100 microns) in the present invention. Smaller dimensional coatings may be used but thicker coatings are preferred in order to accommodate the higher transmission optical densities of the present invention. The highest D-max for the transferred image that is listed in the publication of 1.2. The authors note that when the toners are dried to remove essentially all of the dispersant, the transferred image is of poor quality, with only about 60-70% of the toner particles being transferred.
U.S. Pat. No. 2,930,711 discloses an electrostatic printing method in which liquid developer is used. The dispersant is "blotted" away before transfer of the image, during which process as much as 20% of the toner particles are transferred to the blotting material. The liquid-free powder image is then transferred to a paper coated or impregnated with a thermosoftening material by heat and pressure, or the dry visual image is brought into contact with an adhesive covered transfer media. As is known in the art, both of these methods of transfer depend upon "tackiness" of the receptor coating in order to achieve transfer of the toner particles. Adhesive transfer techniques may result in images having problems of durability. Such images are subject to rubbing-off. In contrast, receptor coatings of the present invention are not necessarily "tacky" but achieve transfer of toner particles due to the critical rheological properties of the receptor coatings. Also, whereas the two above-mentioned publications disclose considerable loss in toner particles (if dry transfer takes place), the present invention transfers at least 90%, and preferably at least 97%, of dried toner particles to achieve images with superior optical densities.
It is well known in the art to use dry powder toner to develop a latent electrostatic image. U.S. Pat. No. 2,855,324 discloses thermoplastic coated receptors to which a dry toner image may be transferred by contact under pressure. As mentioned above, this type of transfer may result in problems of durability. U.S. Pat. No. 3,640,749 discloses coating a transferred dry powder image and receptor with a dispersion of a synthetic resin in water. U.S. Pat. No. 4,071,362 discloses use of a receptive styrene-type resin on a thermally resistant base film to fuse with thermoplastic coated dry toner particles (i.e., image-fixing is achieved by use of a special toner). U.S. Pat. No. 3,620,726 discloses the use of pigment developer of particle size within the range 0.2-30 microns, preferably within the range of 5.0-10.0 microns, with not more than 50% of the particles being of less than 1 micron equivalent spherical diameter, thereby reducing background stain.
The present invention provides a stable electroradiographic, magnetographic, or electrophotographic image of superior optical density, clarity, and resolution, by overcoming transfer and fixing problems often present in the prior art, as noted above. The practice of the present invention is not limited to toner of particular thermoplastic or rheological properties, but depends upon encapsulation of particles in a receptor layer of critical rheological properties.